High resolution transmission soft X-ray microscopy of deterioration products developed in large concrete dams

In concrete structures, the reaction of certain siliceous aggregates with t he highly alkaline concrete pore solution produces an alkali-silicate gel t hat can absorb water and expand. This reaction can lead to expansion. crack ing, increased permeability, and decreased strength of the concrete. Massiv e concrete structures, such as dams, are particularly susceptible to the da mage caused by the alkali-silica reaction because of the availability of wa ter and because massive gravity dams usually do not contain steer reinforce ment to restrain the expansion. Both the cement hydration products and alkali-silica reaction products are extremely sensitive to humidity. Consequently, characterization techniques that require high vacuum or drying, as many existing techniques do, are not particularly appropriate for the study of the alkali-silica reaction becau se artefacts are introduced. Environmental scanning electron micrographs an d scanning electron micrographs with energy dispersive X-ray analysis resul ts demonstrate the effect of drying on the morphology and chemical composit ion of the alkali-silicate reaction gel. Thus, the impetus for this research was the need to observe and characteriz e the alkali-silica reaction and its gel product on a microscopic level in a wet environment (i.e. without introducing artefacts due to drying). Only soft X-ray transmission microscopy provides the required high spatial resol ution needed to observe the reaction process in situ. The alkali-silica rea ction can be observed over time, in a wet condition, and at normal pressure s, features unavailable with most other high resolution techniques. Soft X- rays also reveal information on the internal structure of the sample. The purpose of this paper is to present research, obtained using transmissi on soft X-rap microscopy on the effect of concrete pore solution cations, n amely sodium and calcium, on the product formed as a result of alkali attac k. Alkali-silicate reaction (ASR) gel was obtained from the FURNAS Dam in M inas Gerais, Brazil. Images of the ASR gel in sodium hydroxide indicated di ssolution and repolymerization of the silicate into a less dense form, demo nstrating the expansive nature of the gel when exposed to alkalis. Ln the c alcium hydroxide solution, ASR gel. silica fume, and chemical grade silica gel each reacted with the calcium ions in solution to produce a calcium sil icate hydrate precursor with a lathlike, branching morphology. The distinct ive spherulitic microstructure formed during this reaction was identified a s the 'sheaf of wheat' morphology, previously described in the literature, In addition, the development of the sheaf of wheat morphology nas documente d over time. These results suggest that of the cations studied in this inve stigation, it is the alkalis in concrete pore solution that produce the exp ansive ASR gel, while reaction with calcium ions does not result in expansi on or damage to the concrete structure. More broadly, these results demonst rate the advantage of transmission soft X-ray microscopy for the study of t he alkali-silica reaction, indicating the value of this technique for furth er studies in concrete technology.